Benzaldehyde is an important starting material in various chemical syntheses including those relating to the synthesis of scents and flavors. In these applications, the benzaldehyde is often required to have a high degree of purity, but unfortunately crude benzaldehyde and especially benzaldehyde prepared by the oxidation of toluene with a gas containing molecular oxygen will contain certain impurities that are very difficult to remove. One very significant problem presented by these impurities is that it is particularly difficult to obtain a product from such crude benzaldehyde that will satisfy olfactory specifications. Furthermore, the presence of such impurities also causes a quite rapid discoloration of the benzaldehyde during storage. Such discoloration will occur even at very low concentrations of the impurities, such as a few p.p.m. by weight.
One suggested solution which appears in Japanese Patent Publication 24.467/74 is to purify the crude benzaldehyde by treating it with an aqueous solution of sodium hydroxide. However, this method of purification does not give satisfactory results as shown by the fact that benzaldehyde treated in this manner is still found to discolor quite rapidly.
One method which does give satisfactory results is that disclosed in U.S. patent application Ser. No. 952,609 filed on Oct. 18, 1978. The process disclosed in that application employs an oxidizing agent and distillation to accomplish the purification of impure benzaldehyde.
Still another method of purifying impure benzaldehyde is disclosed in a sister application of the present application filed on the same date in the U.S. Patent Office and which issued on Aug. 25, 1981 as U.S. Pat. No. 4,285,777. There the solution was found to be the treating of the impure benzaldehyde with hydrogen in the presence of a hydrogenation catalyst followed by a distillation step.